The present invention relates to a magnetic recording and reproducing apparatus that reproduces a recorded signal with reduced distortion, and is particularly useful when applied to an apparatus for reproducing an information signal recorded on a magnetic tape by helically scanning the magnetic tape with a rotary head.
A conventional magnetic recording and reproducing apparatus records information data at a high density and reproduces the recorded data by helically scanning the magnetic tape.
In the conventional data recording and reproducing apparatus the information data is modulated for recording by, e.g., an 8-9 modulation method. A record signal thus obtained is equalized by an equalizing circuit, amplified by a recording amplifier circuit, and then supplied to a rotary head mounted on a drum.
The drum is wound with a magnetic tape so that the magnetic tape may travel in an oblique direction. Consequently, the rotary head may helically scan the magnetic tape.
With this data recording and reproducing apparatus, information data is typically recorded at a data rate of 88 Mbps!, corresponding to a maximum recording frequency of 44 MHz!. On a recording track of the magnetic tape, a magnetizing pattern is formed which is reversed at a minimum interval of 0.9 .mu.!.
In this type of data recording and reproducing apparatus, a rotating speed of the rotary head and a travelling speed of the magnetic tape are variably controlled to produce a relative speed between the rotary head and the magnetic tape in the direction of the recording track of 1/1, 1/2, 1/4, 1/8, 1/16 or 1/24 times a normal speed. Therefore, information data having data rates of 88, 44, 22, 11, 5.50 or 3.67 Mbps! can be recorded, corresponding to record signals having maximum recording freguencies of 44, 22, 11, 5.50, 2.25 or 1.84 MHz!.
Specifically, with respect to information data recorded by a record signal having a maximum recording frequency of 44 MHz! and a data rate of 88 Mbps!, the relative speed between the magnetic tape and the rotary head in the recording track direction is variably controlled at a speed of 2 times a normal speed. This information data is readable as information data having a data rate of 44 Mpbs!i.e., a maximum reproducing frequency of 22 MHz!. A low speed reproduction mode at 1/2 times normal speed is thereby attainable.
Conversely, with respect to information data recorded by a record signal having a data rate of 22 Mbps! and a maximum recording frequency of 11 MHz!, the relative speed between the magnetic tape and the rotary head is variably-controlled at a speed of 1/1 times a normal speed. The information data is readable as information data having a data rate of 88 Mbps!, i.e., a maximum reproducing frequency of 44 MHz!. As a result, a high speed reproduction mode is thereby attainable at 4 times normal speed.
In the case of the conventional data recording and reproducing apparatus, variable-speed recording is, as described above effected at a speed of 1/1 through 1/24 times the normal speed. Therefore, with respect to, e.g., observation data which varies slowly as in the case of astronomical observation data, the information data is recorded at a data rate as slow as 3.67 Mbps! and reproduced at a data rate as high as 88 Mbps!. The data may thereby be efficiently analyzed in a short time by using, a computer system.
In contrast with this, with respect to measurement data or observation data which varies quickly, the data can be recorded at a data rate as high as 88 Mbps! and reproduced at a data rate as slow as 3.67 Hbps!. The data can be more thoroughly analyzed at a low playback speed. With this arrangement, a data recording apparatus, shown in FIG. 1, is usable as a buffer for converting the frequency of the information data containing a large amount of information.
In a reproducing system of the thus constructed data recording and reproducing apparatus, an electromotive force e, induced in the rotary head during reproducing, exhibits a differential characteristic expressed by the following formula: ##EQU1## where N is the output of the rotary head. Supposing that a frequency characteristic of the magnetic tape is flat, the electromotive force e exhibits a characteristic rising at 6 dB! per octave.
If a preamplifier disposed within the drum and comprised of a voltage type amplifier circuit having a given noise level is used to amplify the reproducing output, then the signal-to-noise ratio (S/N) of the signal output from the preamplifier decreases with a decrease in the frequency of the signal.
As illustrated in FIG. 1A, in the reproducing system of such a data recording and reproducing apparatus 1, the information data recorded on a magnetic tape 3 is read by helically scanning the magnetic tape 3 with a rotary head 2 having an electromotive force eH. A head output voltage V0 thereby obtained is amplified by a preamplifier 4 comprised of a voltage type amplifier circuit and then supplied as a reproducing Output V1.
Where the voltage type amplifier circuit is employed in the preamplifier 4 for performing variable-speed recording and reproducing in the manner discussed above, the carrier-to-noise ratio (C/N) of the reproducing output V1 has a characteristic as shown in FIG. 2.
More specifically, when the C/N in the shortest wavelength used for reproducing is 40 dB!, measured in a frequency band of 100 KHz! at the fastest relative speed, i.e., a 1/1 times normal speeds the C/N is reduced by 6 dB! as the relative speed between the magnetic tape and the rotary head is slowed by factors of 1/2, 1/4, 1/8, 1/16 and 1/24. As a result, particularly at the 1/16 and 1/24 times normal speeds, the reproducing output V1 can not be satisfactorily obtained.
Therefore, in the data recording and reproducing apparatus 1 capable of variable-speed recording and reproducing at all the relative speeds listed above, it is impossible to use such a voltage type amplifier circuit for the preamplifier 4.
For this reason, in a data recording and reproducing apparatus which has a variable-speed recording and reproducing mode, a preamplifier comprised of a current type amplifier circuit having an input impedance of approximately "0" is advantageous in terms of its C/N.
FIG. 1B shows a reproducing system of such a data recording and reproducing apparatus 5 in which information data recorded on the magnetic tape 3 is read by the rotary head 2 having an electromotive force eH. A head output current I1 thus obtained is amplified by a preamplifier 6 comprised of a current type amplifier circuit and then supplied as a reproducing output V2.
The electromotive force eH of the rotary head 2 has a characteristic which rises at 6 dB! per octave. Correspondingly, an impedance Z of the rotary head 2 exhibits a characteristic which rises at 6 dB! per octave, as expressed by the following formula: EQU Z=WL (2)
Consequently, if an input impedance of the preamplifier 6 is set to almost "0", the frequency characteristic with respect to an input current becomes substantially flat. As a result, the C/N does not vary depending on the frequency.
The amplification characteristic solely of the preamplifier 6 comprised of the current type amplifier circuit has a characteristic which rises at 6dB per octave. Hence, a noise characteristic TN, illustrated in FIG. 3A, appears due to the input impedance noise and the like. A lower input impedance is advantageous in terms of noise caused by the preamplifier 6. Thus, the preamplifier 6 comprised of the current type amplifier circuit is more advantageous in terms of the C/N than the preamplifier 4 comprised of the voltage type amplifier circuit.
In actual magnetic recording and reproducing operations, however, a magnetic conversion characteristic of the rotary head 2 exerts an influence. For this reason, as illustrated in FIG. 3B, an amplitude characteristic of the reproducing output V2 from the preamplifier generally increases in a long wavelength region.
Therefore, if the S/N and the dynamic range are considered, including the effects of the driver of the rotary transformer from the next and subsequent stages of the preamplifier 6 within the drum, and when trying to secure a signal level for the reproducing output V2 of at least 56 dB! greater than a noise level N1 of the preamplifier 6 when the noise level N1 is 100 .mu.V!, then the reproducing output V2 requires a signal level of approximately 63 mV! at the shortest wavelength .lambda. of 0.9 .mu.m!.
When the signal output from the rotary head 2 has a wavelength of 18 .mu.m!, the reproducing output V2 of the preamplifier 6 has a signal level of approximately 2 V!, as can be seen from FIG. 3B.
However, in view of the current capacity of the preamplifier 6 within the drum and the electric power of an integrated circuit, it is difficult to provide a signal level as high as 2 V!. Therefore, when using the preamplifier 6 comprised of the current type amplifier circuit there is a problem with respect to the S/N, namely, that the dynamic ranges of the amplifiers of the next and subsequent stages cannot be attained particularly with respect to low frequency components.
Furthermore, in the thus constructed data recording and reproducing apparatus, the frequency band used for a recording signal changes among the frequencies of 44 to 1.84 MHz! as the data rate of information data recorded as described above changes.
Therefore, it is necessary to equalize the reproducing signal using an equalizing characteristic which depends on the frequency of the reproducing signal, to ensure compatibility with the respective data rates on the magnetic tape.
To accomplish such frequency dependent equalizations, the data recording and reproducing apparatus as described above includes a plurality of equalizing circuits having equalizing characteristics corresponding to the frequencies available for the reproducing signal. The equalizing circuits are selectively used by means of a branch circuit 21 shown in FIG. 4.
In the branch circuit 21, via an input buffer 22 and an input resistance R1, a reproducing output is applied to a transmission line having serially disposed connection nodes to which input selection switches 23, 24 and 25, each comprised of an analog switch, are respectively connected. The transmission line is terminated at one end of a terminating resistance R0, the other end of which is connected to a ground.
The input selection switches 23, 24 and 25 are selectively controlled so that the reproducing signals SA, SB and SC supplied from each output terminal of the input selection switches 23, 24 and 25 are inputted to equalizers (not shown) which have respectively different equalizing characteristics.
Let T be a delay time for each line between adjacent connection nodes. Then, the signals SA, SB and SC supplied from the input selection switches 23, 24 and 25 are respectively expressed by the following formulae: EQU SA=Ee.sup.j.omega.t ( 3) EQU SB=Ee.sup.j.omega.(t+T) ( 4) EQU SC=Ee.sup.j.omega.(t+2T) ( 5)
If the line and the terminating resistance R0 are mismatched, then the signals SA, SB and SC are respectively distorted, as expressed by the following formula: EQU SA=Ee.sup.j.omega.t +kEe.sup.j.omega.(t+4T) ( 6) EQU SB=Ee.sup.j.omega.(t+T) +kEe.sup.j.omega.(t+2T) ( 7) EQU SC=Ee.sup.j.omega.(t+2T) ( 8)